1. Field of the Invention
The present invention relates to a printing apparatus printing an image on a sheet.
2. Description of the Related Art
There is an inkjet type printing apparatus using a continuous sheet rolled up in the form of a roll. Japanese Patent Application Laid-Open No. 2008-222419 discusses an apparatus in which, on the upstream side of a print unit in which a plurality of printing heads are arranged, there is provided a decurling unit configured to remove curl from a continuous sheet.
At the time of its manufacture, a continuous sheet in the form of a roll is pulled under a fixed tension as it is rolled up on a core. In an ordinary manufacturing method, the pulling force applied at the time of rolling up is larger at the end portions than in the central portion of the sheet in the sheet width direction, so that, in some cases, the entire length of the product sheet is larger at both end portions E than in the central portion C. As illustrated in FIG. 1, when compared in a certain unit region, the sheet length L at the end portions is larger than the sheet length I in the central portion. When a sheet whose length thus differs from place to place is drawn out of a roll R, the sheet is straight in the central portion C, whereas, at the end portions E thereof, the sheet is not straight but corrugated due to the surplus length. However, depending upon the sheet manufacturing method, the length of the sheet may be larger in the central portion than at the end portions. In this case, a sheet is corrugated at the central portion. Such corrugation, which is particularly noticeable in a continuous sheet, can also be generated in a cut sheet, which is previously cut in a predetermined length.
It is hard to eliminate such corrugation even by passing the sheet through a decurling unit as discussed in Japanese Patent Application Laid-Open No. 2008-222419. When a corrugated sheet passes through a printing unit, it is possible that the upper portions of the waves come into contact with the printing head. Even if the contact with the printing head does not occur, the distance between the printing head and the sheet surface varies in accordance with the corrugation, so that the requisite flying time for the ink discharged from the printing head to impinge upon the sheet varies, making it rather difficult to perform accurate image printing.